Iran said it would review its suspension of uranium enrichment after a tough UN resolution sharply rebuked Tehran for not cooperating fully with nuclear inspections.
Enrichment, a process of purifying uranium for nuclear power plants, can also be used to make atomic weapons. Any resumption would provoke a major crisis.
Mr Hassan Rohani, secretary-general of Iran's Supreme National Security Council, said Iran would probably not resume enrichment for the moment, but might start building parts for enrichment centrifuges.
The United States says Iran's nuclear programme is a front for building an atomic weapon, but Iran insists its ambitions are limited to generating electricity.
A spokeswoman for the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), declined comment on Mr Rohani's statements.
Mr Rohani said Tehran's decision was prompted by the three European powers reneging on what he described as their commitment to have Iran's case closed at the board level of the IAEA by June.
Mr Rohani said Iran had no secret uranium enrichment sites and the country would continue cooperation with the watchdog.
He dismissed accusations Iran had an undisclosed site in north Tehran, as Washington alleges from satellite photographs.
A UN resolution yesterday "deplored" Iran's failure to cooperate fully with IAEA inspectors.
The IAEA has been probing Iran since August 2002 and has pushed it to be fully open with U.N. inspectors as they struggle to reduce the spread of weapons of mass destruction in an increasingly unstable Middle East.